1. Field of the Invention
The present invention relates to a method of producing a carbonic acid diester.
More particularly, the present invention relates to a process of continuously producing a carbonic acid diester from carbon monoxide and a nitrous acid ester by using a new type of catalyst having a high selectivity, at a high reaction rate with a high selectivity and stability over a long period of time.
The carbonic acid diester is very useful as a starting material of medicines and pesticides, and as an intermediate compound of a polycarbonate and urethane.
2. Description of the Related Art
A conventional method of producing a carbonic acid diester by a reaction of phosgene with an alcohol is well known, and has been practiced for some time. Nevertheless, this conventional method is disadvantageous in that phosgene has an extremely strong toxicity, and thus, is not preferable as a starting material in view of environmental and health considerations. Also, the reaction of phosgene with an alcohol produces hydrochloric acid as a by-product, which corrodes the reaction device.
Therefore, there is a strong demand for a new method of producing a carbonic acid diester without using phosgene.
In response to this demand, various attempts have been made to produce a carbonic acid diester from an alcohol and carbon monoxide, as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai, JP-A) No. 60-75,447, and 63-72,650, and Japanese Examined Patent Publication (Kokoku, JP-B) 63-38,018.
In those methods, the carbonic acid diester is produced by a catalytic oxygen-oxidizing reaction of carbon monoxide with an alcohol in a liquid phase, in the presence of a catalyst consisting of a copper halide or palladium halide.
These methods are disadvantageous in that, in the catalytic oxygen-oxidizing reaction, carbon dioxide is produced as a by-product, and thus the production of the carbonic acid diester is effected with a low selectivity based on the amount of carbon monoxide supplied to the reaction system. Also, the catalytic oxygen-oxidizing reaction produces water (H.sub.2 O) as another by-product, and thus the resultant carbonic acid diester must be isolated from the water by a difficult refining procedure. Also, the above-mentioned methods are further disadvantageous in that since the reaction is carried out in a liquid phase, the resultant carbonic acid diester must be separated from the catalyst in the reaction system.
Accordingly, the above-mentioned methods are not satisfactory for industrial use.
There have been attempts made to eliminate the above-mentioned disadvantages, and as one such attempt.sub., Japanese Unexamined Patent Publication No. 60-181,051 discloses a method of producing carbonic acid diester by a catalytic oxidizing reaction of a nitrous acid ester with carbon monoxide, in a gas phase, in the presence of a catalyst composed of a solid platinum group metal or compound thereof carried on a solid carrier and an oxidant in an amount of 10 molar % in terms of O.sub.2, per mole of carbon monoxide present in the reaction mixture.
In this method, the oxidant in the above-mentioned specific amount based on the carbon monoxide effectively inhibits a production of oxalic acid diester as a by-product, but the addition of the oxidant in the above-mentioned specific acid based on the amount of the carbon monoxide cannot completely inhibit the production of the oxalic acid diester, and therefore, the desired carbonic acid diester is produced with an unsatisfactorily low selectivity. Also the reaction rate and the durability of the catalyst are unsatisfactory.
Further, the above-mentioned method is further disadvantageous in that the proportion of nitrous acid ester in a reaction mixed gas comprising the nitrous acid ester, carbon monoxide, alcohol and oxygen is higher than an explosion (flammable) limit of the mixed gas, and thus this reaction mixed gas is not preferable in view of the safety of the procedure.
Accordingly this method is not satisfactory for practical use.
Japanese Unexamined Patent Publication Nos. 3-141,243 and 4-139,152 disclose a method of producing dimethyl carbonate by a catalytic reaction of carbon monoxide with methyl nitrite in a gas phase by using a catalyst comprising a carrier, for example, activated carbon, and a catalytic member selected from compounds of platinum group metals, for example, palladium chloride and palladium sulfate, and compounds of metals selected from iron, copper, bismuth, cobalt, nickel and tin, and carried on the carrier. This method is unsatisfactory in catalytic activity and durability of the catalyst for practical use.
Japanese Unexamined Patent Publication No. 4-89,458 discloses a method of producing dimethyl carbonate by a catalytic reaction of carbon monoxide with methyl nitrite in a gas phase, in the presence of a catalyst comprising a carrier consisting of, for example, activated carbon, and a catalytic member selected from compounds of platinum group metals, for example, palladium chloride and palladium sulfate, and compounds of specific metals selected from iron, copper, bismuth, cobalt, nickel and tin, and further in the presence of a small amount of hydrogen chloride, while maintaining the catalytic activity of the catalyst at a high level over a long period of time. However, the practical life span of the catalyst is not always satisfactory, and thus there is a demand for an improved method satisfactory for practical use.
As mentioned above, the conventional methods of producing carbonic acid diester from nitrous acid ester and carbon monoxide are unsatisfactory in the durability of the catalyst and are not always satisfactory in reaction rate and selectivity.